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Domain Name System

• DNS stands for Domain Name Service or Domain Name Server (depending on whom you ask).
   
• DNS is implemented on the Web as a set of interconnected routers.
   
• DNS translates a logical URL into an IP address.
   
• Example:
   
  
  http://www.depaul.edu
   
  
  is translated to
   
  
  http://140.192.23.180
  
  
   
  
• Here are three DOS commands for determining IP addresses. Open a DOS Command Prompt window
   
  
  1. To determine the local IP address of your PC:
      
     
     > ipconfig /all
     
     
  2. To determine the IP addresses and port numbers of all active connections to your PC:
      
     
     > netstat -n
     
     
  3. To determine the IP address of a URL, use the ping command, for example:
      
     
     > ping www.depaul.edu
     
     The ping command is supposed to send an IP packet to another IP address and wait for it to return. However, if the destination website is behind a firewall, the ping may not be completed and time out.
      
• The parts of this logical Uniform Resource Locator (URL):
   
  
  http://www.depaul.edu
   
  

  http	Protocol
  edu	Top level domain (TLD)
  depaul	Second level domain
  www	Web server name

  
   
  
• Some common TLDs:
   
  
  com    edu    gov    mil    net    org
   
   
  
• Each TLD is assigned at least one dedicated router that contains the official list of registered second level domains under that TLD. These registered second level domains are administered by ICANN (Internet Corporation for Assigned Names and Numbers) which was created in the early 1990s.
   
• Many countries have their own TLD:
   
  
  au    ca    de    fr    uk  
   
• The Hobbes' Internet Timeline contains interesting historical facts about the internet, including the registration dates for many country TLDs.
   
  
• More URLs:
   
  

  www.yahoo.com	A famous search engine site
  encarta.msn.com	A Web server that does not start with www
  www.bbc.co.uk	A name using four parts rather than three
  ftp.microsoft.com	An FTP server rather than a Web server

  
   
  
• A URL can also contain folders and files:
   
  
  http://ectweb2.cs.depaul.edu/ssmith/proj1/proj1.htm
  
   
• Steps to display a static web document in a browser:
   
  
  • User enters URL of web document in browser. 
     
  • Browser gets IP address of server from DNS. 
     
  • Browser sends request (HTTP GET command) for web document to IP address of server. 
     
  • Server sends web document back to server. (The IP address of the user was contained in the request packet.) 
     
  • Browser displays web document.